When a person does get symptoms, they usually do not appear until 2 to 15 days after the person got the virus. (This is called the virus's incubation period.)[7]

West Nile virus can cause many different types of illness. The rarest, but most serious types affect the neurological system - the brain and the nerves. The most common type of illness caused by WNV is West Nile fever, which does not affect the neurological system.

Neuroinvasive disease is when the West Nile virus infects the central nervous system (the brain and spinal cord). This happens in less than 1% of people who get West Nile virus.[8] However, in those people, it can cause very serious problems. These problems include:

West Nile meningitis:Meningitis is inflammation of the meninges, the layers that protect the brain and spinal cord. People with meningitis usually have a bad headache, a high fever, and a stiff neck.[11]

West Nile meningoencephalitis: This is inflammation of both the brain and the meninges (encephalitis and meningitis).[11]

West Nile poliomyelitis: This causes the arms and legs on one side of the body to suddenly become weak or paralyzed. The person may not have any other symptoms of WNV infection. Sometimes, the breathing muscles can become paralyzed, and the person becomes unable to breathe.[11]

West Nile reversible paralysis: This also causes weakness or paralysis on one side of the body, but it almost always gets better.[12]

At first, a medical professional usually makes a possible diagnosis of West Nile virus based on a person's symptoms, where they live, and when and where they have traveled. The World Health Organization says that doctors should suspect West Nile virus if a patient:[1]

There is no known treatment or cure for WNV. There are no known anti-viralmedications (medicines which kill viruses) which will kill the West Nile virus. Doctors can only provide "supportive care," which means they can only treat the symptoms of WNV. For example, they can give fluids through a needle placed into a vein if a person is very dehydrated; help the person breathe if needed; and give medications to prevent them from getting any other infections.[1]

The West Nile virus stays alive in nature by getting spread back and forth between birds and mosquitoes. Birds get WNV more often than any other living things. When a mosquito bites a bird that has WNV, the mosquito gets the virus. Then, if the mosquito bites a bird that does not have WNV, the mosquito can give that bird the virus. In this way, birds and mosquitoes give the virus to each other and keep it alive.[20]

Humans usually get West Nile virus by being bitten by mosquitoes who have gotten the virus from birds. Humans can get the virus from many different species of mosquitoes.[20]

Though this is not common, a person with West Nile virus can give another person the virus. This can happen if:

There is no vaccine that will prevent West Nile virus.[1] The best way for people to protect themselves against West Nile virus is to protect themselves against mosquitoes. There are many ways to do this. For example:[26]

Staying in places that are air-conditioned or have good window and door screens

Emptying out standing water (water that does not move) from things like buckets, gutters, and flowerpots (mosquitoes like to lay eggs in standing water)

Being extra careful between evening and morning, when the mosquitoes that spread West Nile virus are most active

Scientists can trap mosquitoes and birds, and test them for West Nile virus to see if the virus is spreading in a certain area.[27] If it is, they can do things like spray pesticides to kill mosquitoes.